The present invention relates to a method of producing lyocell-type cellulose fibers by processing a spinnable solution of cellulose in an aqueous tertiary amine oxide according to the dry/wet-spinning process.
In the past few years, a number of processes have been described as alternatives to the viscose process, processes in which cellulose is dissolved in an organic solvent, a combination of an organic solvent and an inorganic salt or in aqueous salt solutions, without the formation of a derivative. Cellulose fibers produced from such solutions were given the generic name of lyocell by BISFA (The International Bureau for the Standardisation of man-made Fibers). The term "lyocell" as defined by BISFA means a cellulose fiber obtained from an organic solvent by a spinning process. The term "organic solvent" as defined by BISFA means a mixture of an organic chemical and water.
Yet, to date, only a single method for the production of a lyocell type cellulose fiber has found acceptance to the extent of actual industrial realization, namely the amine oxide process. The preferred solvent used with this method is N-methylmorpholine-N-oxide (NMMO). For the purposes of the present specification, the abbreviation "NMMO" is substituted for the term "tertiary amine oxides", wherein the term NMMO additionally denotes N-methylmorpholine-N-oxide, which latter is preferably used today.
Tertiary amine oxides have been known to be alternative solvents for cellulose for a long time. From U.S. Pat. No. 2,179,181 it is f.i. known that tertiary amine oxides have the ability to dissolve high-grade chemical pulp without derivatization and that from such solutions cellulose molded bodies, such as fibers, can be obtained by precipitation. U.S. Pat. Nos. 3,447,939, 3,447,956 and 3,508,941 describe further methods of preparing cellulose solutions, with cyclic amine oxides being used as the preferred solvents. In all of these methods, cellulose is physically dissolved at elevated temperatures.
In the applicant's EP-A-0 356 419, a method is set forth which is preferably performed in a thin-film treatment apparatus in which a suspension of the shredded pulp in an aqueous tertiary amine oxide is spread in the form of a thin layer and transported over a heating surface, wherein the surface of that thin layer is exposed to a vacuum. As the suspension is transported over the heating surface, water is evaporated and the cellulose can be dissolved, a spinnable cellulose solution being hence discharged from the Filmtruder.
A method of spinning cellulose solutions is known fi. from U.S. Pat. No. 4,246,221. According to this method, the spinning solution is extruded into filaments through a spinnerette, which filaments are passed across an air gap into a precipitation bath in which the cellulose is precipitated. In the air gap, the filaments are stretched, thus enabling favorable physical properties, such as improved strength, to be imparted to the fiber. By precipitating the cellulose in the precipitation bath these favorable physical properties are fixed, and thus no further stretching will be required. This process is generally known as the dry/wet-spinning process.
In accordance with U.S. Pat. No. 4,144,080, the freshly spun filaments can be cooled with air in the air gap. Further, it is suggested to wet the surface of the filaments with a precipitating agent so as to reduce the danger of adhesion between the filaments. Yet, a disadvantage of such wetting is that the cellulose on the filament surface is precipitated, which renders it more difficult to adjust the properties of the fibers by stretching.
EP-A-0 648 808 describes a method of forming a cellulose solution, the cellulose ingredients of the solution comprising a first component made up of a cellulose having an average degree of polymerization (DP) of 500 to 2000 and a second component made up of a cellulose having a DP of less than 90% of the DP of the first component in the range from 350 to 900. The weight ratio of the first to the second component should be 95:5 to 50:50.
Applicant's WO 93/19230 improves the dry/wet-spinning process and enhances its productivity. This is effected by a particular blowing technique using an inert cooling gas, wherein the cooling is provided immediately below the spinnerette. In this way it is possible to markedly reduce the adhesiveness of the freshly extruded filaments and thus spin a denser filament curtain, i.e. to use a spinnerette having a high hole density, namely up to 1.4 holes/mm.sup.2, whereby the productivity of the dry/wet-spinning process can of course be considerably enhanced. Air having a temperature between -6.degree. C. and +24.degree. C. is used for cooling the freshly extruded filaments.
Applicant's WO 95/02082 likewise describes a dry/wet-spinning process. With this process there is used a cooling air having a temperature between 10.degree. C. and 60.degree. C. The humidity of the supplied cooling air is between 20 g H.sub.2 O and 40 g H.sub.2 O per kilogram.
WO 95/01470 and WO 95/04173 by the applicant describe spinning methods employing a spinnerette having a hole density of 1.59 holes/mm.sup.2 and a spinnerette having a total of 15048 holes, respectively. In each case, the cooling air has a temperature of 21.degree. C.
WO 94/28218 quite generally suggests using spinnerets having 500 to 100,000 holes. The temperature of the cooling air is between 0.degree. C. and 50.degree. C. The person skilled in the art can gather from that document that the moisture lies between 5.5 g H.sub.2 O and 7.5 g H.sub.2 O per kilogram air. Hence this creates a relatively dry climate in the air gap.
WO 96/17118 also deals with the climate that prevails in the air gap, stating that the climate ought to be as dry as possible, namely 0.1 g H.sub.2 O to 7 g H.sub.2 O per kilogram air, at a relative humidity of less than 85%. The temperature proposed for the cooling air is 6.degree. C. to 40.degree. C. The person skilled in the art hence gathers from this literature that the climate during spinning is to be kept as dry as possible.
This can also be gathered from WO 96/18760, which suggests a temperature within the air gap of between 10.degree. C. and 37.degree. C. and a relative humidity of 8.2% to 19.3%, which results in 1 g H.sub.2 O to 7.5 g H.sub.2 O per kilogram air.
Applicant's WO 96/20300 i.a describes the use of a spinnerette having 28392 spinning holes. The air within the air gap has a temperature of 12.degree. C. and a humidity of 5 g H.sub.2 O per kilogram air. Hence, the tendency of keeping the climate within the air gap rather dry and cool, particularly when using a die with a substantially increased number of spinning holes, i.e. when spinning a relatively dense filament curtain, can be gathered from this literature, too.
WO 96/21758 is likewise concerned with the climate to be adjusted in the air gap, suggesting a two-step blowing technique using different cooling airs, and using a less humid and cooler air for blowing in the upper region of the air gap.
One drawback of using low-humidity air is that such air can only be conditioned at a certain expense. Considerable technical means are necessary in order to provide major quantities of low-humidity cooling air for the amine oxide process.
Also, it has been found that the cooling air becomes increasingly warmer and more and more humid as it passes through the filament curtain, since the freshly extruded fibers emerging from the spinnerette exhibit a temperature of more than 100.degree. C. and a water content of about 10% and give off heat and moisture to the cooling air. The applicant has in fact found out that with very dense filament curtains such increasing uptake of water can lead to the situation that the necessary climate can only by adjusted through technically complex blowing devices and that without such devices the filament density cannot be further increased.